We had our introduction to packrats at Lost Dutchman State Park near Phoenix in April, 2010. We camped there one night after arriving in late afternoon. The van had been running fine when we arrived but when I started it in the morning the engine was missing badly.
I used the GPS to search for auto services and started limping to them as I didn't have a phone. After a few duds we came to an old two-bay gas station with a lot of vehicles sitting around. The bays were full but the owner said he'd be happy to take a look and was confident he could find whatever's wrong. In a few minutes he showed me the chewed off fuel injector wires and damage to the wire loom and connectors. He said the biggest problem was the connectors but thought he might be able to get what he needs from the junked cars out back.
I walked back through the junked vehicles with him and I remember him lifting the hood of one to show me the engine compartment of that one was completely filled with packrat nest.
The repair cost me $150 but I was very happy to pay it. I was out of there in only an hour and a half and felt very lucky to have gone to that shop.
The other piece of advice I read somewhere was to look for packrat activity and park in an open area as far away as you can. I remember trying to do that in Nevada a few years ago and it seemed like I was surrounded by packrats-- stacked debris everywhere. I parked in the middle of the parking lot (rather than near the picnic table/fire ring), opened the hood, and put an LED headlamp in the engine compartment overnight.
My latest encounter with varmints was this summer near Red Feather Lakes, Colorado. I had seen a chipmunk about and happened to see it go under the truck and then climb up behind the front wheel and disappear up into the engine area. I checked the engine compartment, blew the horn, started the truck, etc. I saw it a bit later on the rocks nearby, taunting me. (Not the best time to have left the slingshot in the other truck!) I again did the open-hood, headlamp-in-engine-compartment thing.
I also remember seeing this advice from Mr. Pack Rat. It's oriented toward vehicles parked at home but may be useful for its concepts:
Best Bets to Prevent Pack Rats From Attacking Your Vehicle
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