Normally when you sleep, you will use any clothing/blankets in the same square to keep warm, but in a vehicle bed, this does not seem to work.
https://github.com/CleverRaven/Cataclysm-DDA/issues/6887 thanks for reporting.
I thought it was assumed you were storing the whatever in a drawer under the bed, not on it.
I agree with Ryndren here, I don’t think being unable to sleep on clothing placed into the bed of a vehicle is a bug.
Either you put the stuff in a drawer all the time, or you lay it flat on the bed (meaning light items, like clothing, would fly off when you started driving).
It seems like it would just be altogether better to have placed items on a vehicle bed always be in the bed instead of trying to distinguish between the two.
… I’m 99% sure this has already been fixed, and has been fixed for almost a month now.