[Synchro909]

If you only usually blow a 20 amp fuse but not always, it is obvious to me that the solenoid draws about 20 amps It takes a resistance of 0.6 of an ohm to get that (R=V/I or 0.6=12/20) meaning that on 6 volts, the current draw will be 10 amps. (0.6= 6/10)
The soleniod consumes 60 watts of power (P=VxI) P=6X10=60 watts
12 volts is twice 6 volts so the resistor will have to share 50% of the voltage and power. What you need is a resistor of 0.6 ohms and a power rating of at least 60 watts. That is, the same as the solenoid.
I don't know what the resistance and power rating is of the resistors sold by the vendors but they are made for the horns. I suspect the power ratings would be similar but you can connect a number of them in different ways to get just about whatever resistance you want at whatever power rating you want. (I have not had much success with them on a horn - maybe because of quality issues and their ratings being dodgey)