You should have two brown and grey wires coming from the firewall plug (see pic) #1. The run to a plug that fits the switch on the proportioning valve (see pic #2).
If a prior owner cut the wires and discarded them along with the plug, I think your best bet is to look for an older Jeep (any model) that might have the same plug. If no Jeeps in your local Pick & Pay yard, look at other Chrysler products, or any other brand. Jeep used GM steering columns and Ford ignitions. Who knows what other brands and/or models they used to make parts for these old heaps.
Good Luck, L.M.