Objective Good seat belt fit and positioning is important for crash protection. Older drivers experience problems in achieving good seat belt fit and often reposition seat belts and/or use seat cushions. Comfort influences these behaviours. This work examines the impact of functional morbidities on belt positioning and accessory use and whether comfort mediates this relationship Methods Mediation analysis was used to examine the relationship between morbidities affecting physical function, comfort, belt repositions and seat cushion use among 380 drivers aged 75 years and older. Results Musculoskeletal morbidities increase the likelihood of seat belt repositioning (OR 1.37, 95% CI 1.12–1.67) and comfort partially mediates this relationship (P = 0.03). Morbidities of any type also increase the likelihood of seat cushion use (OR 1.15 95% CI 1.04–1.27), but comfort plays no role in this relationship (P = 0.87). Conclusion Greater awareness among older drivers is needed, to ensure behavioural modifications do not impair their crash protection.