1996 Mercedes-Benz C280 Question: ac and heat dont work on my 1996 Mercedes C280

My ac and heat does not come out , it comes on and i see the temp reading but nothing comes out.fan not blowing. someone said i can tap on the blower motor, but i'm not sure where that is located. Took it to auto zone they checked the fuses and the were all good. Is there a relay that i can reset.Also my gas guage does not work unless i go over a speed bump or when i first crank the car up then it goes back to empty. i set the miles now and try not to go over 200 miles per full tank. how can i fix this, what kinda gas should i be using.