This Lepy is a stereo car amplifier. YOu have several tasks to complete:
- check whether your existing head unit has audio outputs. If not, you will have much more issues;
- find reliable, fused point where to get power for your amplifier;
- disconnect back speakers from the head unit,
I know you want to go cheap.. But there are good chances you'll either need new head unit, new battery, or new car. Audio shops around the town are just to make sure you'll get nothing of these.