[Silentalero]

YES size matters.

ground, you can strip the wire and connect it to any metal bolt thats in the car basicly and it should ground it out.

Wire size YES it matters big time becase if you change the siez of it, there is a change of overloading the wire and starting a fire. Most likely the neon wires are 18-22awg (awg = gauge of wire aka thickness of the wire inside)

Now when you get into gauges (awg) of wire the higher the number, the smaller the wire so remember that. If you want thicker wire youd go down in numbers. But for your project you wanna keep it about 18 awg the thickest. Thats what I run throughout my car and it works fine with no risk of fire.

Now the few things to remember, ALWAYS wire correctly, now you can either connect the wires a few ways

Solder the wires (solder kits are about $5-$10) and its basicly a needle point metal pen that heats up and you get a thing of lead that you melt onto the 2 wires to make sure they are secured (takes some getting use to)

Crimp the wires (crimpers can range from $10 on up) and this is basicly a lug/terminal concept where you take some male and female terminals and strip the wire and you crimp the wire into the terminal and its secured. This is my preferred method seing as you can disconnect it if you ever need to for any reason.

Ghetto way (twist & electric tape) I dunno why im telling you this but some people will twist the wires together and electrical tape them or "cap" them, which this is the absolute WORST thing you can do so I wouldnt recommend it incase you were thinking to do this. Its very unsafe becasue you have to remember that the wire your using is a power wire and that wire must be covered and if the electric tape comes off for any reason or those wires split up and the power wire touches a ground your gonna blow a fuse and/or your car may become a fireball.

MAKE SURE YOU FUSE THE POWER!!!!!!! no matter what I cant stress this enough. goto autozone and tell them to give youa fuse holder and fuse for the neons and they should be able to give ya what you need. The fuse goes on the power wire about 12" or less away from the batter or power your tapping off of.