A little update. I was able to make me a little Model 3 racing wheel using a hacked Dreamcast HKT-7400. It works well with the platform's drivers but you're pretty much stuck using auto transmission on most games, and this wheel has no pedals. But it's a great and easy wheel to mod for game collectors who want to play Model 3 stuff.
To use this wheel you open it up with the 4 bottom screws, then as long as you twist the red and black wires (respectively from the 3-pin/steering and 9-pin connector) for 5v, and both your grounds from steering and 9-pin, you should be able to wire up the pots and buttons going directly to the Model 3 filter board.
The two 5v wires need to be tied and connected to both the 5v signals on CN1 and CN11 of the Model 3's filterboard (pins 5 of both connectors). You need to do the exact same for the ground (pins 7 of the connectors). So far I've only tried it with Harley, which for that game you'll need to tie the rear & front brakes together or else one of the brakes goes full on when the other is depressed.
After tying these connections, buttons and analog inputs should connect straight-through. I'd recommend tracking a wiring harness from a Daytona 2 or ScudRace cabinet as it will likely have the amount of connectors and wires you need to wire up your controls. I use a set of crimp connectors, solder seal connectors, and 22-24 awg wire for this. Wire strippers that support as small as 28 awg (the size of wires the DC Racing Wheel uses), a standard crimper, and heat gun are recommended too.
I've got a Daytona 2 ROM board & security chip as well, but as I'm waiting on a replacement CPU Board, I cannot play that right now.
You'll need to be careful when doing this mod as I've had the
wires come off the lugs of the steering potentiometer, and I can honestly tell you soldering it back on there can be a pain since if it's not soldered on there well enough, your steering will likely be janky unless you insulate it well.
And just a safety reminder:
NEVER EVER(!!!) solder wires when your game is powered on by the way. Total shock hazard.
Here's a little wiring diagram I found for most Model 3 games. I forget who made this but it was on Arcade Otaku:
