Relays are like switches. They come in single pole, double pole, triple etc. This is how many circuits it can switch. Then each pole is either single throw or double throw. Single throw has just "normally open" contacts (only closed when the relay is energised). Double throw has both normally closed and normally open contacts (3 contacts with the "wiper" being connected either to the NO or NC contact). Single throw is sometimes called "form A" and double throw is sometimes called "form C". Some relay coils are polarized but many are not. You need to make sure the contact rating of the relay can handle the motor load.

Jim H is talking about using double pole relays.

You would probably be better off finding the original wiring.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra