I recall there is a relay in the 258 equipped Jeeps that handles the carb heater. Take a look at the wiring diagram from any '81 or later Jeep vehicle with a 258 and you'll find the circuit.
I have the same dangling wire on my '77 J10 with '85 CJ manifolds. It's not connected. I expect my truck would run better in cold weather if I connected it, but it's low on the priority list. The coolant circulating through the manifold will keep the carb warm, once the engine is up to temperature.
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk