My son bought a 2005 grand cherokee (which i might add we told him not to buy!) and when he got it home the horn, tail lights and windows would not work. Any ideas of what this could be or suggestions to fix????

 

horn and tail lights could very well be just a bad fuse.
the wiring in the front door commonly break inside the door sock. If you remove the driver side lower kick panel, you should be able to see the wire loom coming in from the door.....lightly tug on each wire and see if any come out with your fingers. The larger black wire is the primary ground wire for that door.

 

Thanks. Will try that and let you know.