Won't rev: Throttle position sensor v wiring harness v ECU

1993 SCL 2L-TE 2wd turbo diesel

Hi all!

THE PROBLEM: when I put the van in drive or reverse, it inches on it's own but is unresponsive to throttle (no revving in park, neutral, or in gear. Engine was rough, but grounding the throttle position sensor smoothed it out.

A mechanic replaced the throttle position sensor, the throttle body.

Using a multimeter, there is an apparent short from the tps wiring harness to some unknown terminal between the transmission and floorboard.

Another Hiace acquaintance suggests the ECU may be the culprit, but I'll have to tow it home to work on the ECU as the mechanic is claiming he doesn't want to be liable for ECU damage.

Additionally, can I access the wiring above the transmission through the floor rather than by taking off the transmission?

Thanks everyone!