2003 Volkswagen Jetta check engine light

My wifes 03 wolfsburg jetta 1.8t runs fine not over heating or running to cold the temperature guage works ok. The check engine light came on so we took it to a local mechanic we deal with since the warranty was up who repairs mainly vws. He pluged it into the computer and it told him the car was running to cold, (i cant remember the code #) he didnt think that was the problem reset the light gave us the bill and said call if it happens again. It happened again 3 more times and the airbag light came on also. Took it back to dealer they looked at it and said it needs a thermostat not sure about the airbag and a break light switch. Then gave us this gynormous estimate and said it but no guarante. Took it to another vw specialist who also put it on his computer and he said it could be alot of different things. He changed the thermostat reset the light 5 days later its back on this time it reads coolant performance problem. Took it back he changes the coolant sensor resets the light. 2 days later its on again. I took it back he reset the light touched the battery cables together and hooked them back up. 3 days later light is on again and now the headlight warning and the keys in the ignition bell doesent work. He is telling me he said this could hapen because it could be anything. I would love to give a donation but all this has cost me over $500. And im farther behind than I was 2 monthes ago. Im laid off and on my income I cant afford thes kinds of repairs. Could you please help? If so great. If not thank you any way. Tracy