On 1987 cars anyway, the fuel injection wiring harness is a complex beast, and it doesn’t much like going through the hole in the firewall and the rubber grommet fitting snugly.
To persuade it to move into position better, BMW fitted some rubber handles to the wiring harness so that you can go inside the car and use these to yank the wiring harness into the car better.
In retrospect, I’d have used some water-based lubricant to ease the way. Once dry, it would no longer have been slippery.
It helps to have person A shove the wiring into the hole from the outside while person B is inside the car, tugging on the handles.