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Don't do that. Based on all the charts I've seen the 06 IS has the Gen5 navi hardware and the 06 GS430 has the older Gen4 hardware. The disc won't work in both.
Correct. The 2006 IS features Gen 5 nav. Your 06 GS430 has Gen 4 nav. The nav discs are not backwards compatible. You will need to upgrade your maps with the correct DVD version for your Gen4 nav system.