Please Please Please, invest in a carbon monoxide detector for your truck... You can buy them at Walmart for around 35 bucks.
I did, and it saved my life... and my husbands... We were starting to get very tired All of the time, and feeling sick a lot. I started having bad chest pains, and extremely sore arms, hands, back pains etc... It turns out that my chest pains, were not heart attack chest pains, but carbon monoxide was starting to affect my lungs!!!
I'm short, so I have to sit up as far as the seat will go... so all of that stuff was blowing right in my face, the whole time I was driving... Anyway, we had it fixed... it was a simple thing really... a loose fitting (Band) around the exhaust, that caused the fumes to come in through the ac/heater vents... but it could've killed us.