The two "slits" you mention are for the removal of the head unit itself.

I don't think that the Becker 1492 & up allow you to remove the faceplate, but I may be wrong.

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1994 E500 (MY SUNDAY DRIVER)
1993 190E SPORTLINE LE W/ M104 SWAP
1997 C230
2002 ML320
2000 BMW 528I (WIFEY'S CAR)

"Excuses are crutches for the unfounded."