Christian,

Fabry, Etude sur les opérations de l'Empereur (from 28th August to 3rd October, 3 volumes) gives numerous proofs, for example letters of Ney who warned Napoleon immediately after his take-over of the command oft the "Army of Berlin" that the Saxon loyality is decreasing [1]. Ney proposed to use the Saxon field troops as fortress garrisons only, but in vain.

Best regards, Thomas
[1] One cause was that the theater of operations was mainly Saxony, which was devastated by both sides. The Saxon soldiers were described to be especially furious to see their home villages looted by their own allies...