Are you talking about a "high torque" starter?

Picture provided by Drudis @ Corral.net/forums

I put a '99 V6 starter on my '88 due to my bigass stock starter slowly dying from heat soak and killing my battery. When I replaced the starter I had to run a wire from the new starter's solenoid to the starter solenoid on the firewall. I remembered a friend's '93 Mustang GT having this solenoid on his firewall so I bought one for my car from Advance/Discount Auto Parts. GP Sorensen part # SS94 which has this second terminal at the bottom (I'm pointing at it in the pic).