My father passed away and his then wife placed his ashes in a memorial wall against his wishes. His wife quickly remarried and the other family members wish to obtain the ashes and dispose of them in the method dad wanted. His ex wife refused to sign a release for his ashes. Dads next of kin is his younger brother. Who has legal say over his ashes ?

The heir of the estate of your father would have given his ashes to the legal heir.

Often this will be his wife

What did the will say.
You said, his wishes, so in his will, did he specifiy on how they ashes were to be disposed of. If he merely told someone, that is not legally binding and has no legal standing.

Agreed, wills trump wishes if different.
And if there was no will, laws vary by state, but the wife gets at least half.
Some funeral homes give ashes to the person who paid for them, or the executor of the estate if there was a will. It's then up to the executor to follow the terms of the will.

Who owns the wall? Ask them what sort of document they need to get possession of the ashes.