Hi i'm restoring my 1979 Malibu classic. I want to use a electric fuel pump using a relay. my question is wich color of wire i must use and where can i find it.
It
sounds like you're still running a carburetor--though I cannot be certain--and you want to transition from using the mechanical fuel pump to using an electric in-line "clicker" pump instead. If I am correct in my guess, that means you'll be using one of the inexpensive generic pumps you can buy at any O'Reilly/AutoZone/etc store, kinda like this random Google image:
I had to use one of these electric pumps after my engine swap because my V8 block does not have a provision for a mechanical pump. I mounted the pump to the underside of the trunk, behind and above the rear axle, near where the OEM rubber fuel line from the gas tank joined to the metal tube that runs inside the frame rail. Two drilled holes plus a couple of hose clamps and it was installed--very easy. For the ground connection, I crimped an eyelet to the pump's black wire and attached it to one of the pump's mounting bolts. Simple.
Like you, I wanted to power this pump via a relay. I mounted the relay under the hood on the passenger side inner fender. You can see that relay (next to another one) in this photo:
Now all you need to do is wire in the relay. In my case, I wanted the pump to be on whenever the key was in the ON or RUN positions. To do that, I found an open slot in my car's fusebox that was hot in those conditions but not in OFF. I plugged in a small wire (either 16 or 18 AWG), added a small fuse (2 or 3 amps), then ran this wire through the firewall to one of the trigger connections on the relay. The trigger ground was easily connected to a nearby piece of metal. To power the pump, I went directly to the battery. Here I used a slightly larger wire (14 AWG), placed a 10 amp fuse right next to the battery connection, and ran it to the relay's power input. For the relay's power output, I also used the larger gauge wire, ran it inside the frame rail to the rear of the car, and connected it to the pump's power wire. For added safety, I then carefully encased all the power wires in black convoluted tubing.
So long as you are familiar with wiring in general and relays in particular, this is a simple operation.