I just went to Radio Shack and got the stuff to make it. When I asked for the 10k Ohm resistors, they had 1/2 watt and 1/4 watt. Which one should I use?, I bought the 1/2 watt.
Also , what is a good gauge wire to use?

Thanks

 

1/4 watt would have been fine, but 1/2 will work also.

I use whatever wire is on my desk at the time. The current draw for the MAX232 or 2 transistor circuits are small, I'd say that anything 20 or bigger (smaller numerically) would be fine.

 

Doesn't matter with the resistor, either the 1/4 or 1/2 watt version will be fine. The wire gauge isn't that critical either. I'd probably use something around 20-22 AWG. Larger is just excess bulk, and smaller might be a bit fragile for an item that might get tossed around in the car a lot.