The slot itself can hold a standard din size radio. The problem is that the MID has an unusual shape and actually extends over the bottom part of the slot. So you can't have both. You have to choose one or the other.

If you search this forum you can find pictures of aftermarket radios installed. Most of them are double-din installs.

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[Sold] 2004 X5 4.4i - Sterling Gray - Truffle Brown, Sport, Preminum, Rear Climate, DSP, Sirius, PDC, 132's (19 inch wheel), Comfort Seats
Dynavin, JL Audio Stealthbox, Alpine PDX-5 Amp