Confused using mapped network share and MSI install errors.

Ok a little info first.
The program i'm trying to deploy is rather dumb that it uses packed filenames that exceed the character limit if executed from ANY folder except for a single drive letter.
The install WILL fail if you try to install it from a regular folder.

So manually i mapped a network drive B: to the network folder where the program is and then executed it like : B:\nameofMSI etc ...

Now i'm trying to do the same via PDQ but it keeps failing during he install with error 1603 (cannot find source files ?)

These are my steps : (All as the deploy user which has admin rights in the domain and thus on the domain pc and on the network share)
1.) create the mapped network drive B via : net use B: "\\server\share"

2.) run MSI command via : msiexec /i "B:\filename.msi" ALLUSERS=1 /qn /norestart /log "%USERPROFILE%\Desktop\output.log"

3.) delete the mapped network drive B via : net use B: /delete

The log file gets created but nothing is written into it as it fails before executing the MSI.

The network drive gets mapped and deleted but the msi installation keeps failing as if it never found the mapped drive ? (error 1603)

I don't really understand why doing it as the deploy user it fails to find the install files but doing it as the logged on user seems to work but then the MSI install fails with error 1619.

Anyone can tell me why this doesn't work or how i can do it better ?
Remember the MSI file will only install from a single drive letter like B:\ , doing it from even a folder called D:\t\ will fail as it goes over the character limit.

Many thanks !