I would first try taking an additional ground wire from the aftermarket unit and running it straight to ground.
if this doesn't help, I would re-route the power from the sony out of the factory wiring(thereby bypassing the original wiring (which probably powers the inst dash)), and take it straight to the hot in the fuse box.
I do have an original schematic for that model, if you want it emailed to you;
[email protected]
hope this helps,
chris
nrt racing