Multimeter set to continuity, one probe in each side of the plug, see if you get a tone. Depends if its factory or aftermarket. Some pound in like a freeze plug others are held in place with a bolt or allen screw with a backet that goes into the block.
its factory no allen head holding it ill try the continuity idea and see whats up im assuming it doesnt work because it doesnt start any easyer when its plugged it or the water temp doesnt warm up any faster when its plugged in
could be your relays.. i thought all mine was bad too and i did some experimenting and it turned out to be my exciter wire on the relays so i cut the ends off of it.. put the ends on new wire.. and got me a heavy duty push button switch and now they are manuel, the timer on the wait to start still works. the colder it is. the longer the light stays on and i keep them pressed in. you could take your intake horn off put a jumper between them and the battery one at a time and look at the grid.