I can assure you that mark ups in the car components business are VERY thin
Yes, the bulk of the profit for parts suppliers comes from selling spares.
It is a while since I was involved but back in the 80s when Lotus Engineering were trying to interest Detroit in licencing their active suspension system the engineers were told the competition for a GM mass produced car was $5, dampers $1 each and springs $1 a set of 4. At that time the servovalves alone were $4000 a set. No sale!
It would be inappropriate for me to quote the make, but I was told the manufacturing cost of a few cars by a senior member of one maker and it was quite close to 10% of the UK retail for the cars. The parts cost is small, the R&D, tooling and marketing costs are huge.