it doesnt have one. only car from 2002 and newer are required to have emergency truck release handles. Here is some background info. http://www.consumerreports.org/cro/cars/car-safety/tips-and-advice/trunk-entrapment-406/overview/
There is no trunk release on a 1991 Chevy Caprice.
Trunk right hand side
On a 1991 Mercury Capri, the trunk release is typically located inside the vehicle, near the driver's seat. It can usually be found on the lower dashboard or near the floor, often as a pull lever or button. If it's not functioning, you may also be able to access the trunk manually by using the key or the emergency release mechanism from inside the trunk itself.
Emergency Call - 1991 1-1 was released on: USA: 3 September 1991
Emergency Call - 1991 1-2 was released on: USA: 15 September 1991
Emergency Call - 1991 1-3 was released on: USA: 22 September 1991
Emergency Call - 1991 1-4 was released on: USA: 23 October 1991
Emergency Call - 1991 1-5 was released on: USA: 6 October 1991
Emergency Call - 1991 1-6 was released on: USA: 22 October 1991
Emergency Call - 1991 1-7 was released on: USA: 17 October 1991
Emergency Call - 1991 1-8 was released on: USA: 27 October 1991
Emergency Call - 1991 1-9 was released on: USA: 3 November 1991