I have no idea who did your conversion , or what board they used , so this info may be useless to you , but we use the Antique Automobile Radio boards ,
like you see here ..and blue would be positive for one speaker and purple would be ground for that same speaker..then Grey would be positive for the other speaker and white would be ground for that speaker .
This may not apply to your particular radio , but that's how the boards from AAR are configured .

Greg