No probs with connections as all the ISO fittings are in the car.

I am having a problem getting the metal sleeve fitted properly. The car
is a 2001 Skoda Fabia. Got the old radio out no probs. The new metal
sleeve fits very tightly and doesn't seem to want to go any further -
you know those last 5mm or so.
I'm considering WD40 or similar to provide a little lubricant.
The car has a plastic opening. I'd considered dispensing with the
sleeve but unlike the old radio which has fitting on it, it's the
sleeve that hold the Kenwood in place ie there's nothing on the radio
to hold in place.
Any ideas folks?