Sorry, misunderstood you post, thought you were using a seperate line level converter but you’re not, you’re using the one built into your amp.
In that case the connections are as follows:
White: Left Positive (+)
White/Black: Left Negative (–)
Black: Common Ground (rarely used)*
Gray: Right Positive (+)
Gray/Black: Right Negative (–)
*The only time you will use the “Common
Ground” connection is with some older (pre-
1980’s) factory systems or head units that ground
their speakers to chassis ground. To use this
connection, ground the black wire on the plug to
chassis ground and only connect the Left and Right
Positive plug wires to the factory radio outputs.