Depends on your head unit, with the Kenwoods You can just ground the parking brake wire, Some other brands need need to send an on off pulse but you can purchase chips for this.I haven't looked at the wiring diagram on the TJ yet. Does it have a speed sensor wire to connect to the receiver?
I've read about certain functions being available based on the parking brake, or that you can ground that wire. I think I'd ground it so the radio always thought it was in park...
Looking forward to your review when you are ready.
For example you cant just ground the Pioneer units (it needs to see a timed on off pulse)
in terms of the speed sense signal its nolonger necessary for the Kenwood units, they released a firmware update so that you can run apple carplay wireless without it. they figured GPS was accurate enough. there were ways to baypass it but its just not necessary any more.
These guys released a gps module which would convert it to speed sense signal for you but again this is dated now. for Kenwood units.

