blower motor issue 84 jimmy

[c10 custom]

Hello everybody i am having an issue with the blower motor on a 1984 GMC JIMMY. The blower motor works when 12 volts are put to it. The heat ac fuse
keeps blowing as soon as you operate the fan switch in the cab. Later finding out that the owner hooked the battery cables up backwards. And also wondering about the oil pressure gauge wiring. I have the 1 prong sending unit
installed by the distributor but the gauge reads full pressure ?
any help on this would be great thank you so much