YES!!! use resistors... otherwise they won't last long. You can run them in series but car voltage isn't one of the most stable. I used (6) 1/4watt 330ohm resistors. You can find LED calcultors all over the net. As for the back, I used gorilla glue to put the two pieces together, and then black silicone allllllll over the backside of it. It's probably submersable... however i did drill one small hole at the bottom so that moisture could escape.
Are you guys using leaded or rosin core solder? I've heard rosin core is the way to go as far as automotive applications, but leaded sure is easier to work with. With the rosin I literally have to touch the iron to the solder, and thats with a 100w iron...