I have a small tach i'd like to install in my '90 245 (M47) in the auxiliary hole between the main instrument panel and radio. I'm not totall clear on the wiring.

When looking at the back of the tach: there is a red/blue wire labeled (+) and a black wire labeled (-) both near the bottom. Near the middle there is thick black wire (with spade connector) - I think this might go to the preexisting tach wire already installed in the dash?? Then near the top of the back of the tach is a grey wire extending from the light bulb assembly and another black wire extends from light bulb assembly to aforementioned negative harness.

I think I could figure this out...I just don't want to blow the tach. I'd rather be certain, so I'm looking to the experts here for guidance. Also, anyone know where I can get a replacement bulb? The bulb looks blown.

Thanks so much. Can't wait to be mesmerized by a tach needle as I race through the gears in my speedy Volvo Wagon. I love this car.