Wiring help for trunk mount battery

A LONG time a ago (about 72), an aspiring electrical engineer friend of mine had a 70 Chev with factory dual battery, and the interconnect relay was about the size of a horn relay. I mentioned "continuous duty solenoid" and he decided to sub a Ferd solenoid. WENT THROUGH TWO OF THEM before he was convinced I knew WTF I was talking about LOL. He even tried a dropping resistor in the second one to limit coil current. First one lasted about 10 minutes as I recall, I don't remember how long no2 went LOL

Miss that guy. Died in his early 60's of diabetes