As stated above, that's for the optional Flux Capacitor. There were only 3 trucks that left the factory with this option installed. You happen to be the proud owner of one of those 3.

You'll have to build a new Flux Capacitor, since Doc Brown is stuck in a different, alternative time period and is unable to help.

But today is your lucky day!! I located a DiY Flux Capacitor instructional web page. You'll have to upgrade the nuclear device located above the spare tire mount to accept garbage to make it work...but I'm sure you have the intelligence to do it. DIY Flux Capacitor

If you are able to make it work, go back to 5/7/1993 and force me to leave with you and talk some sense into me...I don't want to make the mistake of marrying that psycho woman.