[MOTORHEAD]

Get a voltmeter (VOM) across the batt terminals when the tender is hooked up and monitor the voltage periodically.
If less than 12, the charger isn't doing its job. but thats not going to "fry" the battery. look for too high voltage (14 + volts) But the same tender works ok on other vehicle.
Need to check charging voltage. First step to figure out whats going on. If voltage is too high or low, disconnect either cable from batt and re-test with VOM.
First guess is car wiring problem. Don't know the wiring on SPF's, Blas has diagrams if you don't, but the nature of these cars is that strange mods by previous owners is always a possibility !