I do not know much about electronics. I wanted to use my old 70's tachometer on the 88' motor, but the old tach connects to the battary, and the new one uses the charging system. My old tach is the 1970's square faced tach which for one year was made for the 88 Merc ADI system, but are rare now ($200) if you can find one. I've decided to go with the instruments with the 88' Merc.
Jason, here are some suggestions:
1. Type in your tach part number into Google and see if you come up with any wiring diagrams
2. Call Joe Poole at Ferguson-Poole for answers
3. Call the parts dept. at Supreme motors. Allen, the owner, knows electronics.
Mercury uses color coded wires:
http://www.mercuryracing.com/techcorners/outboardfaqs.php Black All Ground
Brown Reference Electrode MerCathode System
Orange Anode Electrode MerCathode System
Lt. Blue/White Stripe Trim UP Switch
Lt. Green/White Stripe Trim DOWN Switch
Brown/White Stripe Trim Sender to Trim Gauge
Purple/White Stripe Trim "Trailer" Switch
Gray Tachometer Signal
Black/Yellow Stripe Shorting or Stop Circuit
Pink Fuel Sender to Gauge
Red Unprotected Wire from Battery
Red/Purple Stripe Protected (Fused) Wire from Battery
Red/Purple Stripe Protected (12 Volt +) to Trim Panel Control
Purple Ignition (Switch) to 12 Volt Positive
Tan Temperature Switch to Warning Horn
Tan Temperature Sender to Temperature Gauge
Tan/Blue Stripe Temperature Switch to Warning Horn
Yellow Starter Solenoid to Starter Motor
Yellow Starter to Regulator (Charging Circuit)
Yellow/Red Stripe Start Switch to Start Solenoid to Neutral Start Switch
Yellow/Black Stripe Choke (Enrichener System)
Green with Color Stripe Switch Box to Coil - Striped and Numbered
White/Black Stripe Bias Circuit (Switch Boxes)