Here's a couple of my previous posts. Several posts above cover the most important step - seal entry points. I'd be willing to bet the your previous rig was better sealed.
Whatever you decide to do for treatment, monitor with a couple sticky boards or traps placed along the edge of a wall anywhere you've seen activity. Bait with peanut butter.
Wish there was an easy fix, but it's a multi step process with finding the holes and sealing them up the toughest part. Even after being in pest control for 16 years, and knowing what to look for, it took me several tries before I found all the little holes in my rig. Last spot was the bundle of wires that came up through the floor behind the dash. The factory had foamed around them but I probed around between the bundles with a finger and found a gap in the center big perfect for a mouse to use. Since sealing that up I haven't had any more problems. I still keep a trap set in the main storage compartment, and inside to monitor for activity.
http://www.irv2.com/forums/f258/rode...-401131-2.html
http://www.irv2.com/forums/f258/mice...-387474-2.html