It might also depend on which method you use to wire it.  I didn't need any separate switch, because I wired it through the existing on/off switch, which serves that purpose.  You could use 16 or 18 gauge stranded wire ("hookup" wire) and possibly some spade connectors/crimping tool (or soldering gear), plus you'll need a wire cutter/stripper tool and electrical tape.  If you want to do a pro job, use some heat shrink tubing instead of electrical tape to cover any exposed wire/connectors (need a butane lighter for that, like you might use to light a stove or grill).  The glue gun can be useful to insulate exposed components & also to tack down wires to plastic housing so they don't flop around.
Good luck!

@Marceline,
Below are my photos of the method I used, with the PWM controller mounted in the empty space, speed control knob mounted underneath it, and 3-way switch rewired as on/off switch. (I made these 2 yrs ago, but can't find whether I ever posted them.)
..
I made this composite to identify the existing parts, with notes about how some would be changed, wired, or rewired..

Next shows the rewired components..

The finished arrangement..






The above photo shows the base of the control knob under the PWM control board, and is covered by a wide piece of heat-shrink tubing to prevent shorting out wires on the soldered side of the board.
Next, I used a scrap of pipe insulation plastic foam as a spacer to prop up the board at the right angle..

View from above showing the fit..

Note above (at bottom) where I used heat shrink tubing over the spade connectors where wires are a bit loose, to avoid shorts.

Above are my kits with assorted sizes of heat shrink tubing & connectors, including the flat spade type connectors I used; from amazon..

https://www.amazon.com/dp/B072LTD1LT

https://www.amazon.com/dp/B01335ZEFE

You also get to make the choice if you want to get rid of the top lid closed sensor (the white wire on the top right of BrianZ's first photo).  If replaced with a solid wire you have a not so effective ceiling fan you can use when the top lid is down.  Maxxaire has that "feature", though others on the forum have noted it's not that grand. Slightly moves the air in the cabin though.

I vaguely remember cutting that off with a pair of diagonal pliers.

Correct, @Marceline, that little tab would fit into a small hole you can drill for the purpose you stated; otherwise, tightening the nut should hold it well enough, or add a lock washer.  There's not really enough resistance (mechanical, not electrical) in that little rheostat dial to matter, so either way should work.

Tundra57 said:

@Marceline. I know its too late, but i drilled the little hole. Im a bit anal about that stuff. But as others have said it really takes little force to turn the pot. Did you fit a power switch to isolate the speed controller input?

I used @BrianZ ‘s method and it looks to me like this effectively turns the 3speed dial into an on-off switch. I could be wrong, though, as 12v wiring isn’t my forte. 

hello.  I just talked to Dometic... my 2020 fan is apparently a Dometic 1450, not a Fantastic Fan.  Is it pretty much the same thing, and will this PWM uprgrade work?  Dometic pretty much told me that the Fantastic Fan upgrade kits would not work on my Dometic fan...  I am a bit confused!

so, cold feet on my part with my Dometic 3-speed fan.  I liked the simpler installation of the rheostat previously suggested, but noted that the wattage was probably too low for the application.  I could live with a only a slight improvement in power consumption, and a quieter fan -- and an idiot proof installation.

uxcell 50W 1K Ohm Wirewound Ceramic Potentiometer Variable Rheostat Resistor with Knob

The rheostat does work and is much easier to install. This one is cheaper and the 25w 50ohm specs work just fine. I installed it in place of the 3 way switch and it works great. I did have to enlarge the hole just a bit.

https://www.amazon.com/YXQ-Resistor-Rheostat-Porcelain-Resistors/dp/B081JZGJJH?pd_rd_w=yer1e&pf_rd_p=cb335323-1c34-41e9-9be4-1577c64bd1ea&pf_rd_r=VF04VX8YSNCNC5MGBF3A&pd_rd_r=011f2268-8cb2-4d4c-99b1-567897c98c84&pd_rd_wg=brQ58&pd_rd_i=B081JZGJJH&ref_=pd_bap_d_rp_1_47_t

I just finished this mod in my 2021 320S BD and it worked perfectly the first try.  I replaced the 3-speed switch with a PWM controller and pot, and installed a rotary on/off switch in the unpopulated fuseholder spot, so no new holes were drilled.

If 16AWG gauge wire is called for, that is the minimum size you should use, you can go larger, 14AWG, but not smaller 18AWG stranded wire.
cheers

@YanniLazarus ; Electronics confuse me no end.  But I took out the 3way switch completely when I put in the PWM.  12v power that used to go to the fuse or 3way goes to the PWM instead.  Output from PWM goes to the intake / exhaust switch or the fan depending on where you want to wire in the reverse / off / forward switch.  Best prior to juice going to the PWM.

Regardless, if you still have the 3 way switch wired in with three different resistors it includes, could it possibly be that you no longer have 12V at the PWM board, and thus it cannot work in any kind of pulse modulated format?

Just a note to let those of you losing sleep worrying about newbie mod-man and his fan rewire... success!  Thanks to all for your patience and help. I love the change...

I completed my mod this morning, thanks for all the information!

I started this in July but I grew frustrated when it wouldn’t work, after I reversed my original install it still was dead. I then discovered my lid was down killing the power. 😂 rookie mistake. It was hot, I had a cast on my arm and I needed a break. 

https://www.opolar.com/products/10000mah-rechargeable-battery-operated-clip-on-fan
my boondock fan