You can install deb files manually without using Cydia by following any of the following methods:
Method 1 of installing iPhone deb files:
This iPhone system trick allows you to install deb files automatically by placing them in a specified location inside your iPhone.
1- Connect your Mac or PC to iPhone using your favorite FTP client, and then browse your iPhone file system to "var/root/media/"
2- Create a folder called "Cydia" in "var/root/media/", and inside the new Cydia folder create another folder called "Autoinstall".
You will have:
3- Put any .deb file you want to install in this Autoinstall directory and reboot your iPhone twice. The deb file will be installed and will show up in Cydia as an installed package.
Method 2 of installing iPhone deb files:
This is my favorite method of installing .deb files on my iPhone 4. It is more visual as you can see the installation process, and see if you have any errors during the installation.
More important, it is location independent which means you can install the deb file from anywhere in your iPhone file system, so you don't have to place it in a an Autoinstall directory as with method 1.
However, it is not free:
1- Install a Cydia application called iFile which costs $4.
3- Copy the deb file from your computer to anywhere you like on your iPhone
4- Open iFile and browse your iPhone file system to the location where you just saved the deb file.
5- Tap the deb file from within iFile, and you should get a menu with three options. Tap the "Installer" option to install the deb file.
Again, once the deb file is installed, you will see it in Cydia as an installed package, and you can un-install at any time from your iPhone normally as if it was installed through Cydia just like any other Cydia package.