Id say a very bad chassis earth mate ,
ive lost count of the amount of cars i have found this on ,

Usually after a rebuild or resto where the cables havnt been put on properly / tightly

Id say yours have a bit of corrosion on them

its also well worth checking the main red cable as twin 45's said .

good luck ,

Ps if it behaves similarly if you put the lights on or load the battery id deffo say this was the problem .