I posted
just such a scenario yesterday.
The hardest part I think is getting India to go along, although something like a high profile terrorist attack / assassination could be the catalyst.
If the Pakistanis can be persuaded to throw in the towel early in order to save the Punjab and Sind I think the Indians/Afghans/Soviets could accomplish their war aims at reasonable expense. If, however, the Pakistanis refuse to concede defeat things would get VERY ugly: an invasion of the Pakistani heartland would be a bloodbath for all concerned (which is why I think the Pakistanis would have the sense to surrender and cede peripheral areas first).
edit -- I don't think Pakistan had any working nuclear weapons at this point, and if they did have one or two not-very-deliverable a-bombs they'd be suicidal to use them under any scenario.