Yes as above the fets might need more drive volatage, your car motor is probably only getting a few volts, the hi side looks ok, the issue will be the low side.
You could try 2 more fets on the n channel part of the bridge, and use a pullup resistor to 12v on the gates the same as you've done with the + side, if you do dont forget to invert the drive from the micro as the gate drive fets will invert the on state, ie hi = fets off.
You can get automotive rated fets that will turn on fully at 5v, I've never used such devices.
Another way to get that circuit to work is to use gate drive transformers and rectifier caps on the gates of the fets, the xformer steps up the drive voltage and isolates the gates, so the + side fets can be n channels without extra gate drive devices.
There is a little software overhead, depending on your pwm freq you need to keep pulsing the fets to keep the rectifier cap charged or discharged.