They may be interelated but then again. Here are the areas where I have located grounds:
Batt-Chassis (@ the batt.)
Chassis-engine (mine was attached to an exhaust bracket that worked loose and caused very strange problems-double check this one due to vibration)
Instr. Panel (behind the cluster are many grounding points)
Trunk Area (Structure point on each side, near the lid supports)
The cruise control may be grounding, but double check the tail light sockets (believe it or not). I found the tail light assembly gasket was not sealing well and the bulb socket was corroded. A little emory cloth and things started working again. A little silicone grease renewed the gasket sufficiently.
If not, you may be looking at a little solder touch up on the amp.