OEM head units from model to model?

Nissan does use a standard type of radio plug for most of the '90's vehicles.

Double-DIN basically just refers to the size of the radio. A DIN radio unit is generally two inches high by seven wide; a double-DIN is either a radio four inches high, or two DIN units stacked one on top of another. 90's Nissans often used the stacked DIN units, or a DIN radio on top of a DIN-sized pocket. For the most part, the only Nissan radios without built-in amplifiers are the Bose models; if it's not a Bose head unit, then it should have a built-in amp.

The brackets that hold your existing factory radio can be removed, and attached to another Nissan radio. They do use a standard set of mounting holes; only the brackets differ from one vehicle model to another.

The trouble with Bose systems is that the head unit, amplifiers and speakers are all designed to work together, and don't necessarily match any after-market standard. That means you can't always replace a Bose component with an after-market component and expect good results. The head units don't have standard types of outputs, the speakers aren't the standard impedance, and the amplifiers don't always play nice with new head units or new speakers.

Not sure what difference the model makes as you would use the gain on the amp to match it up anyway.

ALL radios have amplifiers in them to run the speakers.

BOSE are a pain because most utilize a factory external amp for a factory sub on premium packages. They also use different impedances for their speakers than the traditional 4 ohm.