I seem to have installed a tach with a rheostat a little while back...here's the deal...

I have a 1987 245 DL. Found a tach and sm. clock a while back and did the install. The clock worked fine for the frist day, untill of course, it got cold. The next morning it had stopped. Tach worked great. Just as it should. I asked a question here about sm. clock illumination and thusly replaced the bulb. The bulb worked, it lit up when i inserted it into its housing on the clock, but it didn't seem to actually illuminate the clock face. On to the main issue.

While I was unplugging the clock wires, i tapped the red wire end to the black terminal which still had the wire attached. It arced. Ever since then, my tach has been linked to the interior light system. With the lights off, tach works fine. With lights on, the brighter the interior lights, the less sensitive the tach. The tach is entirely turned off with lights turned all the way up. The tach is obviously only getting current throught the clock light circut. I was just out in the car learning all the facts i could about this. If I unplug the clock bulb, I have no tach at all. If I unplug the clock entirely from the tach, I have no tach at all. I had the whole insturment cluster out of the car a week ago when i fixed (removed) the temp. compensation board, and to my visual inspection, all looked normal. What gives?

Is my tach fried? Any help or insight is apreciated. Thank you.

1987 245 DL, 228k (You folks are the best. BB rocks!)