I have a 1999 GS400 that needs a wiring harness replacement, I was wondering if I can use a harness from a different year GS400 or maybe even a LS or SC model?

 

Need to narrow down what you have now these were the variations in 1999

82111-3K540
82111-3K550 W(DISCHARGE HEADLAMP)
82111-3K560 W(DAY TIME RUNNING LIGHT SYSTEM & DISCHARGE HEADLAMP)
82111-3K750 W(DAY TIME RUNNING LIGHT SYSTEM)

 

How can I check which one mine is?

 

Go to your car and check which of those options it has. "Discharge headlamp" refers to HID headlights, btw.

 

Would it be on the driver side door that little sticker with the VIN and everything?

 

If you create a profile on the Lexus driver's website and enter your VIN it will show you everything the car was optioned with.