Laptops dont like water!! Put an expensive Laptop in ya truck and it could end up an expensive choice?
Maybe a look on Eblag and find a cheap option.....Also then if you do leave it in your truck, if some git steals it, you know that it aint gonna be worth much.
Panasonic Toughbooks on Eblag are a good choice. Something like 600MHz processer, 128mb Ram and 20 GB HD with a couple of USB's would set ya back say...£250
Most GPS/Navigation sotware has a low use rate on system resources, so run prettty well on lower end Laptops
Just my two pennies worth on the subject..........