My 300tdi Defender ,used daily ,pulls a trailer with a 180k miles runs like a dream,plenty of power,but smokes in reverse? Any ideas on the smoking

Just fitted seals from Rimmer bros,not OEM but gaps that I never had before. Don't need to open windows anymore. Are OEM seals any plumper to actually fill the gap? Adjusted door catch as much as possible,but in vain... Any help greatly appreciated