Laura,
If the Grand Cherokee has a tow receiver and light connection on the rear you should able to tap into the system there. Other than that, can't help with advice on locating or tying into the existing wiring, but you may want to check out the wiring kits Blue Ox offers; both incandescent and LED versions. The advantage is that the tow light wiring is completely separate from the toad's system. I've installed the incandescent versions on both my Wrangler and Liberty and have been very satisfied.
Steve