if i had a dollar for each time i have heard "im an electricain, i know what im doing"......
Re test the wires in the loom,you have probly got the constant mixed up with the illumination wire.
It would be very very unlikly for the constant to only show 2.8, they will generally ether work fine or not at all.
A quick test to see if you have indeed mixed it up. turn your headlights on and if the head unit starts working (while on acc) then you know you have hooked into the illumination wire.
If nether of you are confident with it dont attempted anything further, just take it to a professional and save your self the headache.
Jack