personaly for a short term measure i would fit a tempoary starter button. probably get away with a bellpush. I would think you could use a 4 pin relay. dont know too much about pin arangement etc so couldnt say for sure.
i take it you have been through all the clutch swiches and interlocks etc as motomartin suggested
looking at the haynes wiring diag for 88-90 GS. conector 30is fed via a feed that is spliced on to a main feed from the battery. so it should show battery voltage unless the splice is failing, the wire is breaking down or the batery conection is dirty. you could try a feed to here from the battery (has it got much charge left)
you tried jumpstarting it this isnt going to turn into the longest diagnosis of a duff battery is it
