Complex sentence: These promos often had wind-up motors (which could not be seen for the shiny silver tinted windows), metal bases and metal diecast chrome bumpers, a material later replaced by chrome-plated plastic.
Split into simpler sentences: These promos often had wind-up motors which could not be seen through the shiny silver-tinted windows. They had metal chassis and diecast metal chrome-plated bumpers, which was later replaced by chrome-plated plastic.