Regarding placing third party experimental payloads such as those from AMSAT, this has been done several times on test launches particularly with Arianespace until recent years. Then the aerospace engineering company became an accountancy conglomerate with a few engineers, and since then free rides have been elusive.
Arianespace gained quite a bit from AMSAT's involvement. Firstly, they pioneered the adapter rings that allow multiple payloads on board Ariane rockets, spreading cost from the prime contractor across secondary payload customers. They also act as an open organisation for platform and integration demonstration for Ariane's customers: most launch customers don't like third parties sniffing around their IP during integration.
Yes, AMSAT have had launch failure, such as the Phase IIIA launch failure in 1980 which saw global news coverage at the time. The reaction to this by AMSAT was immediately to try again. Frankly, if you go into space, and can't accept any risk of failure, you should find something else to do with your life.
The difference nowadays is the degree of risk-averseness, and that includes such things as mitigating against the risk, say, of an in-orbit collision of multiple launch objects. As such, insurers have become key parts of the equation, and the associated costs of that, which can be prohibitive.
The car itself, well that will not be simply a car popped into the faring with a few bolts. It will have been through substantial thermal, vacuum and vibration testing to mitigate against such things as outgassing. I doubt there's much actually inside the body, and what's left will have been highly modified to satisfy the aforementioned testing.