My 2005 Forester is starting to have problems starting when it's been sitting for a while in very rainy or damp weather. If I open the hood and let the sun hit it, it starts up. I had the wires changed about a year ago. I'm considering changing the coil pack (looks like an easy job?), but was wondering if there's anything else it might be?
A good place to start is bad grounding. Clean both battery posts and cables. Look around the vehicle and engine for grounding points. Remove each one you find, clean and re-install. There are a bunch of grounding points so be patient and methodical. As you encounter electrical connectors, unplug them and re-plug them a few times - clears out the cobwebs.
If you have a low-power battery charger, put it on the battery and let it charge overnight.
Hard starts could also be a fuel issue / fuel pump issue. Check power and ground to the fuel pump - unplug and re-plug the connector a few times. You might also run some kind of fuel system cleaner through the tank (Sea Foam is popular).
You have already guessed that hard starts are related to your ignition system. Check the high tension wires carefully. For a 2005, the wires are due for a change. You indicated that this has been done. But if the wires you used are 'brand x', you might replace them again. Get an OEM wire set or a set from NGK. Also, check that the wires plug into the coil pack fully and the boots seat correctly on the coil pack. When I had an 05 X, one of the wires was absolute hell to get onto the coil pack correctly.
While your at it, you might check / change the plugs. Your vehicle has a so-called 'wasted spark' system - NGK makes a 'dual platinum' plug that works nicely with this system. Finally, check your coil pack for connection - both low and high voltage connections. If you remove it from the vehicle, inspect it for tracking or high voltage breakdown. Coil packs are usually reliable, but they have been known to fail. Yes, removing or changing the coil pack is not too hard.
Still on a possible electrical / ignition issue, you might try this. Get a garden hose with a nozzle set to fine mist. Go out at night (when it's dark) and raise the hood, start your vehicle. After it's running and warmed up, spray the engine with the fine water mist. A well ordered out engine should not be affected by the mist. If you have bad ignition wires or sparking coil or poltergeists the results can be quite dramatic.