My 84 Chevy pickup truck was the same way: 6 volt plugs and a controller. I think this was a way to get the GP hot quicker. The previous owner was too cheap to replace a bad controller and wired in a manual switch to replace the controller. I had trouble starting it one day and I burned out all the GP. I found that 12 volt plugs are available so I replaced the bad 6V GP with new 12V GP and haven't had any trouble since. I push the manual switch and wait 10 seconds and it starts almost instantly. I don't need GP at all when engine is warm.

On the older model MB, GP are turned off as soon as the starter is energized. I think some of the newer MB leave them on like your truck to make the engine run better when cold.

Yes, the new pin style GP are 12 volt plugs. The older loop style were 1.2 volts and were in series with themselves and resistors to drop the voltage. Its almost as if the plugs were made for a 6volt electrical system. I could never figure why MB didn't go to higher voltage GP because half the energy was lost in the resistors.

P E H