Bad earth? several faults, flat battery

[Huntington]

After dropping a new engine into my VT i have had several teething issues, the current issue i am working through, I think is a bad earth connection somewhere, but i just cant find it. problems its causing are

engine warning light constanly on, then off then on, no codes left to tell me why
tacho moving up and down at idle, without revs, perfectly smooth idle, moves as high as 5k

now i go out after two days and find my brand new battery is to flat to even crank the engine. nothing at all left on.

and as a real kicker the engine fan is coming on randomly.

im not real electrical minded but all the connections and earths that i can see look good and solid. anyone got any idea of what to look for?

[VK SL 3800]

Its sounds almost as if the ECU is playing up, Theres an earth at the back under the throttle body, the main earth near the oil filter above engine mount, and two more on the front if i remember correctly.