I'm about to install the D3 in my Jeep and after reading several posts I realize that soldering the wires together for the bypass is the best method. However, I'm not too sure what type of solder to buy? Advice and assistance in soldering would be great!

You can get pretty much any kind of solder. The most readily availible is 60/40 rosin core. Availible at radioshack or any other electronics store.

I don't understand how it can be easier IMO, I just used a blue butt connector and joined 3 wires that way.

Solder is best. It retains its bond and excellent conductivity thru vibrations that will shake off other type connections...