Since you're looking to obtain proof that can be used to prove infidelity in divorce proceedings, the very first thing that you need to do is to get more information regarding which methods you can employ w/o being charged w/ breaking state or federal wiretapping laws.
For example, is your state (assuming that you live in the US) a one-party consent state? If so, you can record any conversation in which you are participating w/o informing the other participants that you're recording the conversation. Placing a VAR in his vehicle, though, may prove problematic.
If he uses a Windows or Mac computer w/ any sort of regularity, you might consider using a keylogger, though you may have to be able to show that it's your
computer in order to justify having done that. Not sure if living in a CP state would help you there at all.
You could also place a GPS in his vehicle, but, again, that may not be "legal", strictly speaking.
As has already been mentioned, you should read through @weightlifter
's "Standard Evidence Post" thread. (There is also a similar thread authored by @badbane
You should also start talking w/ attorneys. Again.
Oh, and because I haven't said it yet...