Use a 1.6 : 1 voltage divider to get 5V from the 13V input and then buffer it with a unity-gain opamp to get a low output impedance to drive the next stage:
+13
|
+---------+
| |
[16k] |
| |
+--------|+\
| | >-+-- 5V>
[10k] +--|-/ |
| | | |
| +---|---+
+---------+
|
GND