According to a quick google your car should have an audible repeater if it was fitted with a trailer since the late 80s...
Does your car have a towing box, or is it wired directly into the loom?
If it's got a box, does the box have a buzzer in it?
If there is a 'speaker' on the box is it face up, or face down? (I couldn't hear the one on my car until I turned the box over)
If it's a box, has it got wet at some point? Water can destroy some speakers.