[Ascaris]

A mercury switch is just a little tube with a blob of mercury inside it and two electrical contacts on one end. When the tube is tilted one way, the blob runs down and covers the contacts and completes the circuit. When it is tilted the otherway it breaks the contact.

Only drawback I could see with this arrangement is that the light could also come on if you were to go down a VERY steep hill or rear end somebody. In both cases, your trunk light coming on momentarily would probably be the least of your worries.