To convert 13.8v to 12v I use a adaptor similar to this one:

Actually I have it set for 9V - this is fine both for the camera and for the GPS video overlay unit.
To covert to 5v I use DC to DC Adaptor with USB Outlet:

I assume that old cell phone charger adapter also workes fine but this particular USB adaptor has a very good quality. I decided to use this USB adaptor to protect the DVR.
I also used DC Power Extension Lead similar to this:
It is important to:
1. connect the 5v adaptor directly to 12v adaptor and
2. connect the camera's both leads (+,-) to the output from the 12v adaptor (and not to use the car body for (-)).
3. connect the GPS overlay unit directly to the output from the 12v adaptor.
4. you should also check if the 12v adaptor really gives 12v output. Mine camera actually works better after I switched this adaptor to 9V.
cheers