forswaps

I dont know if anyone has any ideas to help, but my 300 was getting hard to start when cold so as a 2006 model I orderd 6 new glow plugs and they were fitted by my local garage. When I picked the car up all was fine but after around 30miles I had the check engine light come on and the thunderbolt light. These did clear on there own the next time I started up but again after about 30 miles I had the check engine light on again. Using my obd reader i had error codes saying that the glow plug control unit was faulty and also that glow plug 6 with circut open. I fitted a new glow plug control unit and all again was fine for about 30 miles then check engine and red thunderbolt light on and in limp mode again, I then took it back to the garage and they cleared the codes as there was a really long history of codes as well. I drove for again 30 ish miles again check engine light but this time no thunderbolt light I checked with my obd reader and it said map sensor faulty so I replaced this and also glow 6 and 4 open, but as soo as replaced it and on first start it lit up both lights again, so back to garage but when they checked it they did not get that code for plugs as i had cleared all. But they did get map sensor fault show up, so they swapped the air filter one with the engine one on the front and cleared codes, but again after about 10 miles both lights on again, no they say they think it is a wiring loom problem which I feel will cost me an arm and a leg. I had no lights on before any new fitted plugs this has all statred after they done them. Is there any reason anyone can think of, could it be the garage or is it just coincindance .