I started out yesterday inmy 93 xlt 2.3L and the smell of something burning was coming from the vents in the truck. I shut the truck off and looked under the hood and smoke was coming from the blower motor resistor. I was going to replace this again ,but after removing the resistor the area behind the resistor was packed with leaves. It appears the blower motor resistor had heated up causing the leaves to begin burning and that was the burning smell . I cleaned the area free of leaves and replaced the with the same resistor and everthing seems to work fine. I did not replace the resistor because it looked ok and it was replaced back in 04. Hopefully this may help someone. Note: this is the second time this has happened.

 

Can you tell how the leaves are getting in there? Maybe you should look into ways of preventing the leaves from getting in there