Pics from my later (89) car. It has stock Porsche (Alpine) radio. Unit passes through a similar bracket to that shown by OP. Also has a bracket / bar extending from firewall to support bottom / back of unit. 
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Vince
2015 Boxster S, Black & Black
1989 Carrera Silver Coupe, Silk Grey - sold
2009 Cayman S White, Full Cocoa - sold
1972 911E Silver Coupe, Pepita & Black - sold
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