If the trolley has Station Stop effects like the subways (many P2/P3 trolleys do), it can be programmed and put into auto mode without adding DCS hardware.
It's not as simple to do it, as you need to be good with timing the presses of bell and horn buttons, but it is possible.
No gutting required, most likely (unless it is a unit that does not include Auto mode, I'm not sure if they always have or not). The manual should cover this.
-Dave