What amp do i need for my car?

You want something that will put out 400 Watts RMS at 2 Ohm. Preferably the amp be CEA-2006 rated so then you can be sure it will put out that amount of power. If its not its probable putting out less then stated.

If it CEA-2006 and rated above you need a simple DMM

When you set it you want to use a 40 Hz tone with, subs disconnected, HU at 3/4, start turning the gains until you reach 28.3 Volts AC on the DMM.

If its rated just right at what you need then you can do it by ear.

If it just rated below you can do the same by ear just less power..

If you have some money to spare you could get a good amp

http://www.sonicelectronix.com/item_69515_Kicker-K...

If not then this might be better and its just under what you need

http://www.sonicelectronix.com/item_33266_Massive-...

There are some others that meat 400 but they was out of stock and you can look around.

You want the sub wire like this

http://m.seimg.net/product/img/subwoofer_wiring/1_...

First you will absolutely need an amp to power any subwoofers. it may not affect your electrical system too much. It all depends on the RMS wattage your system .