Your query is not clear. Plz elaborate the situation.
Do you want that your manager should go and work there? If the drivers of vehicle are not coming what do you expect your Manager to do ??
Is there anyone in HR or Admin who looks after these matters, or everyone is left to work for himself as they deem fit ??
If he can get conveyance allowance or transport to visit or inspect the working at that place, how can the Manager refuse??
Plz inform him verbally, or if he is required to claim any TA for the purpose then a letter from the Competent Authority can also be sent.
This is a routine matter, according to the nature of activities carried out by your company.
You can also issue an order with the wordings, like....
".....you shall be present at the site at the beginning of shift, to ensure the availability of vehicles and take appropriate action......"
Please also ensure that he gets the usual conveyance, as per your rules, for visiting the site.
Also, how he will mark his attendance, that also must be ensured.
Another point is, some serious punitive action needs to be taken against the Drivers for thgeir lapses in performing their duties.