If you are upgrading to a lighted mirror in your Mercury Capri, you will need to install the wiring. Your Capri is equipped with the wiring socket from the factory. Here's some pictures of the OEM socket location, where the harness plugs in. If you don't have the OEM harness it's possible to improvise the wiring like I've done.
The socket in question is located in the black terminal block above & behind the fuse panel. It's the second 4-pin socket from the right, see pictures below.
Fuse Panel under the dash
Just above & behind
Hope this helps!