Try this
http://www.bgfindashop.com/bgservices/climate.htm , Pat Goss highly recommends this and should solve your problem.
The Lysol work, but seems to be short term, I've tried it as Pat Goss directed.
Run your truck with the heat and fan on high, spay a can of Lysol in the air intake, dont forget to have the doors open.
Also, running the heat and fan on high in a used care can clue you in if its had water/flood damage. The heat and fan will oncover what someone maybe hiding.
Hope this helps.
Erick
2004 DC SR5
debadged
Magnaflow
K&N drop-in