You have 2 choices
1) have an avionics shop install a panel mount intercom
2) buy a portable intercom. These units connect to the existing radio jacks and are not considered installed equipment.
Advantages of 1: clean and neet, intercom is integrated into radio install.
Disadvantages: expensive
Advantages of 2: inexpensive
Disadvantages: the cockpit looks looks an explosion in a spaghetti factory with wires running all over the place. There is no place to plug in the intercom in a older Cherokee so you need to run the intercom off a battery