Here are some pictures that I took a while back when replacing the batteries in the alarm unit in my 2004 9-5. The alarm units on earlier model years are easily opened with screws. Units in later model years are one piece so you'll need to cut them open and reseal again when done.

Definitely wear safety glasses when you cut it open.

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Don't cut too deep or you may damage the circuit board

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#6 x 1-1/2" sheet metal screws worked great to secure the two parts

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Not the prettiest sealing job but as long as it is sealed, you won't see it when it is installed again.

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