2010 Ford Galaxy electric problem

[Sean17]

Hi all I have had problems with my 2L diesel Galaxy starting when I bought it privately. The car would cut out and the traction control light would come on. After putting Fordscan on it and having loads of electrical faults I found out they had put a smaller battery in it which was basically dead. After buying a new battery everything was great until a week ago. The battery was dead again. The engine malfunction light would come on and the car would cut out. 
 

So my question is what’s the best way to test all the electrics in the car as I think something has to be leaking the battery, there are no interior/exterior lights on when the car is off. 
If the car is off no keys in and I put my tester on the fuses would this show any that are energised or not. 
also when driving the engine light will come on and the traction control light, the engine light will go straight off but the traction control light stay and this could happen numerous times. 
 

any help please. 
very grateful 

[Sean17]

I think the glow plugs also need replacing but don’t think this would drain the battery