If the car is running negative earth then keep it that way unless you are contemplating concourse.
No modern cars run negative earth and modern accessories such as radios, phone chargers, GPS and in car video systems are set up for negative earth.

Ref. The Lucas DR3 wiper motor it is almost 100% for sure that the slow run problem is in the cable and wheel boxes.
I have rebuilt heaps of these motors, (the latest in Feb this year) and slow running always comes down to the mechanicals, not the electrics.
When removing the wiper motor it is absolutely essential that you label the wires as the old fly leads to the motor will have lost their colour coding.

By the way that is a pretty unique colour for a MK2!!

 

Bill, you mean "No modern cars run positive earth"
Most here know what you meant, but a new comer may get confused.

 

Apologies
I meant to say no modern cars run positive earth.
Thankyou Jeff