If it is pre wired you'll find the romex if you remove the vent's interior trim piece. My last TH was pre wired to the bedroom, but not the garage.
If it's not pre wired and/or you have 30 amp service or you want to be able to run the 2nd AC off a 5500 generator... see this thread.
Also, even with 50 amp service I would not recommend running a 2nd AC off of a 15 amp interior outlet that's on the house circuit with other loads, It will be a challenge that is only possible to consider if you purchase a high efficiency AC and manage the load.
The best 5500 generator option I've found (other than running romex from the panel... messy!!!) to power a 2nd AC was using a 12 gauge extension cord from the isolated exterior 20 amp GFI outlet to a Marinco 20 amp inlet wired to the AC. (photos attached)
I installed an AC in the garage of my last TH that was not pre wired and have helped two of my friends add AC's to their TT's as outlined in the link above. If you have any specific questions just let me know.