Next let's wire the AC switch and power leads.
In the back of the fridge, cut into the "hot" leg of the 120VAC wire, following the 3A fuse. (Leave most of the length on the end leading into the heater unit, because the Norcold wire is more heat-tolerant than the wire you're probably using.)
If you want to create an "emergency bypass switch" for AC:
Connect the Norcold "hot" lead to the common (middle leg) of an ON/ON switch. This is the kind of switch which supplies power to ONE of the other two legs at a time, never both.
Make a connection from one of the free switch legs, stripping the other end for a red wire nut connection with two other wires. Also strip the "free" end of the wire you previously cut (the Norcold "hot" wire into the heater core). The third wire of the wire nut will be from the SSR. 
Connect the other secondary side of the ON/ON switch to a one of the switched "AC power" terminals of the SSR. Connect from the other switched "AC power" terminal back to the wire nut, and tighten it with 3 wires. (One wire from the SSR; One wire coming directly from the ON/ON switch; and the Norcold wire into the "hot" side of the heatger core.)
How they bypass switch works:
Set the Norcold selector switch to "AC". When you want to run TM-controlled, because "Rick's newfangled thing broke", then you flip the switch to the non-SSR wire: Current reaches the heater wire directly through the "Y". (When bypassing the SSR, you will need to turn down the Norcold temperature dial to an appropriate level.)
But the "new normal" mode is to use the SSR: Flip your switch to the activate the leg which groes through the SSR on the way into the "Y", and turn the TM temperature controller to maximum ("6"). The PID will de-activate the circuit when the temperature reaches it's low limit, and re-activate the circuit when it sees that you are about to reach the high limit.
If you wire directly, with no bypass switch, you simply turn the interior Norcold selector switch to "AC", and turn the Norcold Temperature dial to maximum ("6"). You should double-check to be sure that your DC On/Off switch is set to "Off", because it is possible to run PID-Controlled DC at the same time as propane or AC. Since electrical power from the Norcold dial is either fully "on" or fully "off", DC and AC together creates double the heater power - and might damage your ammonia solution, or even create a crack in the sealed Refrigerator. (DC together with the Propane burner at
less than full power is an intentional use of this design. DC with AC is a
very bad idea.)
- - - - -
My fridge stays between 34.6 and 34.9 degrees (note that I have the thermocouple installed in a hole near the coldest part of the fridge - near the bottom, and closer to the side with the propane heater behind it).