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The only way the ex-wife would be getting the benefits after he died, is if your husband elected a survivorship benefit when he first started taking his pension upon retirement. This is the "normal" way a married person elects to receive a pension. Once this election is made and the first monthly pension benefit is paid, the election can not be changed. Also there is going to be something stated in the property division part of the 1999 divorce decree that confirms the ex-wife's entitlement to the benefits upon his death.
So I am sorry to report that there is nothing you can do to get your husband's pension benefits. The only favorable news is that you might qualify for increased social security benefits using your husband's employment record rather than your own. Since you were married to him for more than 10 years, you would be entitled to spousal survivorship benefits from the Social Security Administration and you should check that out.
Given your recital that the decree says they each keep their respective pensions, you might have a chance to contest the benefits being paid to the ex, if she was named as a beneficiary before the divorce and the divorce said she would not be a beneficiary and after the divorce the husband forgot to remove her from the pension. To get to the bottom of this would require an attorney to review the divorce decree or final orders and would also depend upon whether the GM pension is set up under federal ERISA laws or under state laws. If it is a "federal" pension then it doesn't matter what the state divorce papers say, only who was named on the beneficiary forms with the pension.
Let me know if you need any further input from me.