You see a crash landing near the airport but you're in a car, no other planes have reported the status of the plane, and Approach can no longer contact the plane. Can you report the crash on an emergency or approach frequency to the tower?
If you are in a car, you probably don't have the full picture. Chances are, if the accident happened "near the airport", emergency services are probably already on the way. This is because the pilot would have been communicating with ATC, and would probably have indicated that he was in trouble before the actual accident. Even if the pilot did not have time to notify ATC, if a plane suddenly "dissapears", search and rescus will be initiated.
In any case, if will do no harm to call the normal emergency services (dial 112 or 911 in some countries). Don't overcomplicate things and try to get in touch with ATC - just dial the emergency number directly, and they will know what to do.
Yes, you can. If you have an aviation handheld or a ham radio in your car, you can call the tower. FCC rules allow you to use any frequency in an emergency. The catch is that you are supposed to exhaust other means first, so if you have a cell phone you should call 911 first.
Note that this is only FCC rules. Your local jurisdiction might still arrest you for interfering with police or fire operations if you, say, were to report the crash on the local EMS repeater.
It's doubtful, though, so long as you do what you are told and are concise.
I think you might have been trying to ask "Can you report a crash landing on an emergency frequency from your crashed aircraft even if you are not at the airport, and you don't have a mobile phone or have no mobile coverage?"
It's unlikely an ATC radio facility will be reachable on the ground more than a few miles from an airport, so the Answer would be "yes you can", but your message would need to be received by (high-flying) aircraft in the vicinity monitoring the emergency frequency, and I am not sure many aircraft do so anymore. You would be better off trying to contact an aircraft on an your last used ATC or uncontrolled field channel (like 122.9) that might be intercepted by an aircraft in the vicinity.