[Atomic-S (OP)]

Having discovered that rats have been doing deleterious things to my automobile, I was wondering if they, being nocternal creatures, would be discouraged from climbing into the engine compartment by placing a lamp under the  vehicle all night.

[Don_1]

Hmmm, not too sure what that might do, other than run up your electricity bill.

You might like to give it go and, while you're about it, leave a socket set, screwdrivers, oil and filter to see if they carry out a full service while they've nowt better to do.

[RD]

A changing stimulus is more effective than a constant one :
If possible have the light go on-and-off at random intervals, say for intervals between 5 seconds and a minute , [that will reduce the electricity consumption too during the off periods ].

[Atomic-S (OP)]

I had not thought about a blinking light; that could have advantages. However, it will wear out the bulb faster.

I also have a leaking transmission and power steering system, which I really would like the rats to repair, bur the outlook for that does not look very good.

[CliffordK]

Perhaps use a fan, or something that makes a little bit of racket.  I wonder if a motion detector would pick up the movement of a rat.