I have codes P0300, P0301 and P0305. How do I check the ignition?

Hello, first the Mountaineer never came with a 3.0L V6 engine as you listed. Second, the codes you indicate are misfire codes from cylinders 1 and 5. These are not necessarily indicative of an ignition system issue, but could be. Most people would start by checking the spark plugs and coils and or coil boots to see if spark is not staying with the cylinder as needed while the engine is running. A relatively simple test is to swap 1 and 5 for 2 other cylinders and see if the misfire chases where you put them when codes come back. If it does, then you know you have your culprits. My wild guess is that you need spark plugs and coil boots replaced. However, proper testing can save you a ton of misdirected money. Try to enlist a local shop near you by clicking on this link for further assistance and good luck! http://repairpal.com/mercury-within-10-miles-of-walla-walla-washington-99362