The Pfister Comet S2 from the Los Santos Tuners update converted to GTA IV with tuning parts kept and fully usable, along with a moving steerng wheel and all liveries.
I've included the .max project file for anyone who wants to make their own fully tunable car.

REQUIRES v65.0 or newer of Zolika1351's Trainer/Mod Menu to work, otherwise the car is completely unusable. v66.0 or newer recommended to avoid instability because of the high amount of tuning parts on this car.
PUT COMET6.DAT INTO THE "carmods" FOLDER! (if it doesn't exist, create a folder called "carmods" in your game folder and put the .dat there)

Use https://gtaforums.com/topic/955449-iv-zolikapatch/ to make the steering wheel move and unlock the 4 livery limit

WARNING: This car will be somewhat unstable with anything older than v66.0 of the trainer as it's pushing IV quite a bit with 75 separate tuning parts spread across 7 categories. It's recommended to remove this car from traffic or turn traffic off entirely if you're using an older version.

Vehicles.ide:
comet6, comet6, car, COMET, COMET, VEH@LOW, NULL, 30, 999, 0.2621, 0.2621, 0, 5, 1.0 ,0, noboot+sports+wheelangle:-19.887+livery
txdp
vehshareV, vehshare
race_interior, vehshareV
race_generic, race_interior
comet6, race_generic

Carcols.dat:
comet6, 0,0,63, 1,1,12, 33,33,27 46,46,127, 6,6,8, 49,49,59, 127,127,123, 62,62,63, 22,22,64, 56,56,59,

Tuning part count: 75
Tuning part list:
Stock Front Bumper
Tuner Bumper
Backdate Bumper
Backdate Comet SR Bumper
Backdate Race Bumper
Low Drag Bumper
Performance Bumper
Performance Bumper w/ Carbon
Endurance Bumper
Stock Rear Bumper
Primary Rear Diffuser
Secondary Rear Diffuser
SR Spec Rear Bumper
Smooth Rear Bumper
Backdate Rear Bumper
Vented Smooth Bumper
Vented SR Spec Rear Bumper
Full Rear Bumper Delete
Stock Hood
Secondary Hood
Carbon Hood
Smooth Hood
Secondary Smooth Hood
Carbon Smooth Hood
SR Spec Hood
Secondary SR Spec Hood
Carbon SR Spec Hood
Vented Hood
Secondary Vented Hood
Carbon Vented Hood
Endurance Hood
Secondary Endurance Hood
Carbon Endurance Hood
Stock Wing
High Ducktail
Carbon High Ducktail
Carbon Stock Decklid
Smooth Wingless Decklid
Carbon Wingless Decklid
Mid Level Wing
Backdate Decklid
Carbon Backdate Decklid
Retro SR Wing
Retro SR Wing & Carbon Blade
Tuner Wing
Track Wing
Drift Wing
Street SPL Wing
Race Wing
Endurance Wing
Extreme Performance Wing
Autobahn Racer Wing
GT Wing
Time Attack Wing
Loop Battle Wing
Circuit Master Wing
Retro SR Mega Wing
Retro SR Mega Wing II
Stock Roof
Secondary Roof
Carbon Roof
Stock Skirts
Carbon Street Skirts
Street Skirts
Carbon Skirts
Secondary Sunstrip
Race Skirts
Carbon Tuner Skirts
Tuner Skirts
Carbon Performance Skirts
Performance Skirts
Flared Aero Skirts
Aero Skirts
Primary Sunstrip